Integrated circuit or microcircuit chips are generally mounted on lead frames for coupling to external circuitry. Lead wires are bonded between die pads on the microcircuit chip and the lead frame fingers using manual or automatic ball bonding machines. The fine lead wire is generally stored on a stationary spool. The wire is drawn off the end of the spool and fed through a capillary wire holding tool or bonding tool which performs successive multiple bonding operations. Typically, one end of the lead wire is bonded to a die pad of the integrated circuit chip using ball bonding methods. The other end of the lead wire is bonded to a lead frame finger using a wedge bond or weld.
An example of a manual ball bonding machine is the Kulicke & Soffa Industries, Inc. (K&S), Model #478, which is actually a semi-automatic machine. The capillary wire holding or bonding tool is manually positioned by an operator viewing through a microscope objective. The bonding itself is then performed by an automatic operation of the ball bonding machine. However, manual positioning and monitoring slows down the lead wire bonding operation. A modification and further description of the manual or semi-automatic ball bonding machine is found in U.S. patent application, Ser. No. 294,411, filed Aug. 19, 1981 for LEAD FRAME WIRE BONDING BY PREHEATING, by the same assignee as the present application.
The fully automatic lead wire bonding machine greatly increases the speed of the lead wire bonding and chip packaging operation. An example of the automated ball bonding machine or robot is the Kulicke & Soffa Industries, Inc. (K&S), Model #1419, Hi Speed Ball Bonder. A modification of this ball bonding machine for fully automatic error correcting robot operation is described in U.S. patent application, Ser. No. 470,217, filed Feb. 28, 1983 for LEAD WIRE BOND ATTEMPT DETECTION, filed by the same inventive entity and assignee as the present application.
With the advent of high speed automatic ball bonding machines and robots a difficulty is encountered in feeding the fine capillary lead wire from the source such as a spool to the capillary wire holding and bonding tool. In present bonding machines such as the K&S models referred to above, the capillary bonding wire is stored on a spool approximately two inches (5 cm) in diameter and about one inch (2.5 cm) in length. The spool is mounted in a stationary position and is designed to permit the fine wire to be fed off the end in a direction parallel to the axis of the spool. This has proved satisfactory for the slower operation manual bonding machines. For the high speed bonding machines, however, the length of wire stored on the spool is inadequate requiring frequent replacement. The lead wire is normally stored in a single layer to avoid deformation or entanglement of the fine metal wire which may be, for example, a fine gold wire or copper wire, 0.001 inches (0.0254 cm) in diameter. For storage of greater length attempts have been made to wind multiple layers of wire on the spool. A disadvantage of this method is that forces from overlying layers tends to deform the underlying layers when wire from the outer layer is drawn from the end of the spool.
Another disadvantage of the conventional method of storing and feeding the fine capillary lead wire is that the wire may twist and turn as a result of torquing forces in the wire itself produced by drawing the wire from the end or top of the stationary spool in the direction parallel to the axis. Thus, for example, it is noted that the lead wire may twist and turn in the capillary wire holding bonding tool and interfere in the bonding operations.
A further disadvantage of the traditional methods of storing and feeding bonding wire in the automatic ball bonders results from the high speed operation. The multiple bonding steps at high speed requires that a reserve of lead wire be maintained and available for feeding directly into the bonding tool in steps of rapid succession to match the high speed of the bonding steps. The conventional ball bonding machines afford no such slack reserve of lead wire sufficient for the high speed automatic operation nor can they assure feeding of lead wire at high speed without becoming tangled, or distorted or deformed.